Difference in structural engineer opinions - how to proceed?

boots_babe

Hi,

We are nearing the end of some extension work, but have come up against an issue. When we knock down the external wall to open up to the new extension, we will need an RSJ.

Our own structural engineer has provided calculations and a plan for this. However our builder has since consulted with his 'own' structural engineer who he has worked with for many years and trusts, and they have come up with a very different solution.

Obviously this forum is not going to tell me which is right, but I was hoping for some ideas as to how best to proceed, what questions we should be asking, before making a decision. We need to be confident that the chosen solution will be safe!

Option 1 - knock down the wall, build a pillar at each and end insert an RSJ. The engineer is telling us that the RSJ is actually over spec'd for what is needed as he was being cautious. He has also said that once the wall is down, we must examine the foundations to see what is there, and consult with him, to check his solution is going to still be valid based upon what we find.

Option 2 - as above, but with 2 differences. Firstly the RSJ is a much higher spec, he says the option 1 spec is insufficient. Secondly he says there is no way that the existing foundations will be enough to support the new RSJ. He says we must dig a trench under the length of the wall, between the 2 new pillars, around 2ft wide/45cm deep, and fill with reinforced concrete. However the other engineer strongly advises against digging such a big hole in the footprint of an old building, as he's worried it is likely to cause movement in the rest of the house.

So we are stuck between a rock and a hard place. Both engineers have 30+ years of experience and sound very plausible when they explain their thinking. We need to make a decision today/tomorrow, but need to be sure we have a safe solution. What questions should we be asking, how would you go about working through this?

Any and all suggestions gratefully received, thank you.

rob7475

Did the extension need planning permission or does it fall under permitted development? Either way, your local council's building control should have been involved to approve your plans. If you are now thinking of changing those plans, you need to speak to building control to make sure they are happy with what's proposed.

tacpot12

I'd try to get the two engineers to talk to each other, and see if they can come up with a solution that avoids the risk of movement even if the RSJ spec is higher. Make it clear you respect their view points, but want to avoid risk of movement in the old building. 

Failing that, I think you are into the realms of selecting the engineer with the best indemnity insurance!

Eldi_Dos

Speak to your Architect.

Never been a great fan of "we need a decision by tomorrow" when it comes to changing specs.

boots_babe

rob7475 said:

Did the extension need planning permission or does it fall under permitted development? Either way, your local council's building control should have been involved to approve your plans. If you are now thinking of changing those plans, you need to speak to building control to make sure they are happy with what's proposed.

Yes we had planning permission, and yes we are working with BC in regards to the calcs.

DullGreyGuy

Presumably the lower spec option is from your own engineer whereas the higher spec is from the builders?

stuart45

You need to speak to Building Control about this. Engineers will sometimes have different opinions about how a job should be done. Generally speaking they put a safety allowance into their calcs.

boots_babe

DullGreyGuy said:

Presumably the lower spec option is from your own engineer whereas the higher spec is from the builders?

It is yes. Our builder has been very transparent with us on costs and has worked hard to keep costs down for us in various areas. So it's not that he's trying to get more out of us, if that's what you were thinking.

Annemos

Is Option 2 like what is done when a Building is underpinned after Subsidence? 

I know Engineers worry about damage occurring if only a Partial underpinning is done, rather than underpinning a whole property. 

The 2 parts with differing foundations can move against each other. 

vacheron

I'd ask both engineers if either of them had had a structure collapse or partially collapse in their respective 30 year careers.

If the answer from both is no, go with whichever you prefer. 

I'd also keep Building Control updated with any changes in the plans and provide them with updated drawings as soon as possible. They will confirm if the solution is acceptable as part of their signoff.  

boots_babe

Annemos said:

Is Option 2 like what is done when a Building is underpinned after Subsidence? 

I know Engineers worry about damage occurring if only a Partial underpinning is done, rather than underpinning a whole property. 

The 2 parts with differing foundations can move against each other. 

 I'm not sure sorry, as thankfully haven't had to ever deal with subsidence. But they are proposing a certain size of trench being dug under where the RSJ will be inserted, which will be filled with reinforced concrete, so that the weight of the RSJ is not just on the 2 pillars at either end, but spread across the reinforced concrete area too.